chore: prevent cursor carryover between preload test phases

[Nic-Polumeyv]

data-sveltekit-preload-data fails intermittently under server-side route resolution, about 1 in 6 locally on version-3, and anything that shifts hydration timing makes it worse (a one-line import change on #16308 took it to 3 failed retries in CI).

The test navigates between phases with the cursor still parked over the previous phase's link. When the fresh page hydrates, Chromium fires a mouse event at that position and the router preloads that link's target, so the phase that expects zero requests counts a preload it never triggered. A request timeline shows the target's __route.js, node script and __data.json all arriving before the phase's goto resolves, ahead of any hover.

Parking the mouse before each navigation removes the carryover. 10 of 10 runs green under ROUTER_RESOLUTION=server with the change.

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[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from 9d42896:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/9d42896474c8381f13952d86477ec71fc5fb7829

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16510

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[Rich-Harris]

ah, this is great! we've been meaning to set up an agent loop to tackle these flaky tests as they arise, but it never rises all the way to the top of the pile